1. Field of the Invention
The present invention relates to UV-absorbing poly(organic oxidized silicon) particles having improved UV stability, which include polysilsesquioxane, polysiloxane and a silsesquioxane-siloxane-silica copolymer, and a method for preparing same.
2. Description of the Related Art
Ultraviolet (UV) light is known to negatively affect the skin, leading to burn or skin cancer. To prevent such skin damage, UV blockers containing organic or inorganic compounds that absorb UV and are less toxic to human body are used. Recently, use of UV blockers is increasing even in general-use cosmetics such as lipsticks where they were not used formerly.
At present, 17 kinds of UV blockers are approved in the US and 11 more kinds are approved in Europe. Among them, there are two particle type inorganic blockers: titania and zinc oxide. Organic blockers which are mostly liquid materials are known to penetrate into the skin without exception. Accordingly, inappropriate use may be harmful to the human body.
Since the two particle type inorganic blockers, i.e. titania and zinc oxide, are insoluble to water or general organic solvents, they are considered as safe without the possibility of penetrating into the human body. However, because they exhibit the white turbidity phenomenon of turning white when mixed with other substances, they are prepared into very small size of less than tens of nanometers to prevent the white turbidity phenomenon. However, regulations on nanoparticles smaller than 100 nm in size restrict this approach. Some of the actually used particles are smaller than 10 nm and it is reported that nanoparticles of this size may penetrate into human skin. The photochemical properties of titania change with particle size. Smaller particles tend to absorb rather than reflect light. Thus, whereas titania particles having a diameter approximately between hundreds of nanometers to micrometers reflect UV, titania nanoparticles absorb most of UV and emit electrons, thereby degrading nearby organic substances. Accordingly, titania nanoparticles are used as the so-called photocatalyst that degrades organic substances when UV is radiated. Since a severe problem may occur if the same reaction occurs on the skin, the titania nanoparticles for UV blocking are coated with various materials. However, the white turbidity problem is not solved thereby. For these reasons, titania is used in UV blockers in an amount less than 10% in most cases.
Cosmetics prepared by physically adding titania or zinc oxide to silica particles or polymer particles such as poly(methyl methacrylate) to prevent the white turbidity phenomenon and solve the nanoparticle problem are commercially available. However, the cosmetics obtained by this method have unsatisfactory lubricating and extending properties. When they are used in a larger amount to solve this problem, they cause the white turbidity problem again.
Polysilsesquioxane particles were prepared as one of poly(organic oxidized silicon) particles having p-methoxycinnamic acid groups. Since this particle has very good compatibility with organic and inorganic substances owing to the UV-absorbing p-methoxycinnamic acid group, it provides advantage in appearance to such an extent that petrolatum comprising 30 wt % of the particles with a particle diameter of about 1 μm do not exhibit white turbidity. Further, it provides good UV blocking effect such that a sun protection factor (SPF) of about 13 is achieved when it is included in an amount of about 10 wt %. In addition, it also substantially removes UVA owing to scattering and reflection of light by the particles, as compared to other organic UV blockers.
Despite the many advantages of the polysilsesquioxane particles having p-methoxycinnamic acid groups, they exhibit very low UV stability as compared to the liquid UV blocker octyl p-methoxycinnamic acid having the same UV-absorbing group, because the double bond of the cinnamic acid group undergoes [2+2] cyclization easily when exposed to sunlight or UV from a UV reactor. Although this problem can be solved indirectly to some extent by mixing with another UV blocker, the low UV stability is a very important defect for a UV-blocking substance, greatly limiting its application and commercialization. In addition, since polysilsesquioxane has a rigid crosslinked network structure with one silicon atom connected to another silicon atom via three oxygen atom, it has a rough feel.
Preparation of polysilsesquioxane spherical particles having N,N-dimethyl-p-aminobenzoic acid groups is much more complicated as compared to the spherical particles having p-methoxycinnamic acid groups. The preparation process is complex since the particles are not formed alone and other ingredients should be added together. In addition, the polysilsesquioxane having the N,N-dimethyl-p-aminobenzoic acid group also has much lower UV stability than N,N-dimethyl-p-aminobenzoic acid. Also, particles with rough feeling are obtained like other polysilsesquioxanes.
As such, UV-blocking particles prepared from polysilsesquioxane are difficult to be used for actual application due to the rough feeling and the fatal problem of low UV stability.
Soluble particles may also be used to improve UV stability while avoiding the skin penetration problem. It is known that skin penetration does not occur if the molecular weight is sufficiently large, for example about 1000 or larger.
Throughout the specification, a number of publications and patent documents are referred to and cited. The disclosure of the cited publications and patent documents is incorporated herein by reference in its entirety to more clearly describe the state of the related art and the present disclosure.